1. Field of the Invention
The present invention relates generally to a heat shield for agricultural light bulbs and more particularly, to an air-cooled light fixture having a transparent cylinder that surrounds a high intensity discharge lamp for providing a continuous air path for removal of heat from the fixture.
2. Description of Related Art
High intensity light sources, such as agricultural bulbs, sealed beam lamps and the like, include optical devices which direct the light therefrom along a narrow beam path. These lamps get exceptionally hot when confined to a small area. It is important to operate these lamps at or as near as possible to their recommended operating temperatures. Higher than optimum temperature operation reduces their life span and lower temperatures prevent inert gas contained in many such lamps from returning tungsten from the bulb wall to the filament coils which blackens the lamp walls causing a reduced intensity output.
Several air-cooled light fixture designs in common use have sheet metal enclosures used in combination with borosilicate glass for providing a heat shield to insulate outside surroundings from heat generated by these light fixtures. However, the sheet metal enclosures typically get very hot during operation and radiate substantial heat to the surroundings. Additionally, several of these light fixture designs have channels formed inside the light fixtures that have nonlinear contours, thereby producing increased mechanical losses from airflow through the light fixtures and thereby reducing air-flow capacity associated with the channels. For example, U.S. Pat. No. 4,546,420, issued Oct. 8, 1985 to Wheeler et al. discloses an air-cooled light fixture having an air channel defined by several structural members projecting inwardly to form a jagged-shaped air-path. Such jagged air-paths impair the capacity of an air channel to cool the light fixture by restricting an air flow path. Similarly, German patent number S 41624 discloses an air-cooled light fixture having an enclosure surrounding a lamp with a channel formed in the enclosure for passing air through the light fixture. However, German patent number S 41624 does not disclose the light fixture having a straight and continuous channel for air flow. Consequently, heat removal by an air-cooling means cannot occur at a maximum flow rate. Great Britain patent number 721,585 discloses an air-cooled light fixture having a glass enclosure surrounding a fluorescent tube, wherein connected to a side wall of the enclosure are two conduits serving as an inlet and a outlet for passing air flow inside the light fixture. However, Great Britain patent number 721,585 does not disclose a light fixture having a straight and continuous air-channel configuration permitting maximum flow of air through the light fixture.
Several patents disclose a light fixture having a glass cylinder surrounding a light. For example, U.S. Pat. No. 5,489,813, issued Feb. 6, 1996 to Jung discloses a neon lamp including a transparent outer tube for preventing water from entering the lamp. However, U.S. Pat. No. 5,489,813 does not disclose a means for air cooling the lamp. U.S. Pat. No. 5,612,585 discloses a high-pressure discharge lamp including an outer envelope surrounding a discharge tube and a heat shield. However, U.S. Pat. No. 5,612,585 does not disclose the outer envelop as a heat shield.
A shortcoming of some common air-cooled light fixtures is their inability to sense the temperature of a light fixture and terminate electrical power to that light fixture when its temperature exceeds a predetermined maximum safe operating temperature. Another shortcoming of some air-cooled light fixtures is their lack of adaptability for being connectable to exhaust systems having varying air-flow capacities. Still another shortcoming of some common air-cooled light fixtures is an absence of waterproofing for protecting the light fixture from water damage. Yet still another shortcoming of some air-cooled light fixtures is an inability to be connected to outside sources of ventilation air.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed, nor fulfill the needs or problems as set forth above. Thus an improved air-cooled light fixture is desired.